1. Field of the Invention
The present invention relates to a lens assembly for projecting an image. More particularly, the present invention relates to a lens assembly for projecting an image, in which positions of lens elements can be adjusted finely without lowering image quality of image projection.
2. Description Related to the Prior Art
A rear projection apparatus has been recently known as a display apparatus or TV set of a large display area. The rear projection apparatus includes a screen, a mirror, and an optical engine. The screen is supported on a casing. The mirror is disposed behind the screen. The optical engine is a projector component which projects projecting light toward the mirror. The optical engine includes illumination optics, a light valve, and projection optics. The illumination optics receive light from a high brightness light source to output uniform illuminating light. The light valve is an image forming device, and modulates the illuminating light to produce an original image electronically. The projection optics enlarge and project the projecting light in a manner having image information formed by the light valve.
An aspherical lens element can be included in a lens assembly for the projection optics. To produce the aspherical lens element, raw material is molded by injection molding into the product, examples of raw materials being a plastic material, glass or the like. However, a mold for the injection molding has a gate, which causes a gate mark to remain on the product of the aspherical lens element being plastic typically. Also, weld lines may remain on the injection molding as a flow of the molten plastic material. When one wishes to use of the aspherical lens element as a part of the projection optics, the aspherical lens element should be installed to offset positions of gate marks or weld lines from an effective projecting region of image projection. See JP-A 2004-361620. This is for preventing image quality from being lowered.
There has been a conception of a common use of the optical engine for a plurality of types of the rear projection apparatus different in the screen size, for the purpose of economizing production of the rear projection apparatus by considering the optical engine. The screen size can be changed by changing a distance from the projection optics to the screen. However, initial positioning of the optical engine is determined suitably for a specific size. If a user wishes to install the optical engine in a type of the rear projection apparatus different in the screen size, the focus must be adjusted. To this end, the projection optics are moved in the optical axis direction for the adjustment. To move the projection optics in the optical axis direction, the use of helical threads is effective. However, the gate marks or weld lines on the edge of the aspherical lens element of a plastic material are likely to exist within the effective projecting region for image projection. Also, offsetting of the aspherical lens element changes the degree of image surface distortion of projected images, to lower image quality of the projection.